1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor, and to an image forming method, an image forming apparatus and a process cartridge using the photoreceptor.
2. Discussion of the Background
Organic photoreceptors (OPCs) are widely used instead of inorganic photoreceptors for copiers, facsimiles, laser printers because of their superior performances and advantages. Specific advantages include (1) optical properties such as the ability to absorb a wide range of light and the ability to absorb a large amount of light; (2) electrical properties including high sensitivity and stable chargeability; (3) materials; (4) good manufacturability; (5) low cost; (6) non-toxicity, etc.
On the other hand, as image forming apparatuses become smaller so have photoreceptors Photoreceptors are also required to have good durability since recent image forming apparatuses produce images at a higher speeds and are free from maintenance. In this respect, the organic photoreceptor has a soft surface layer mainly formed from a low-molecular-weight charge transport material and an inactive polymer, and therefore the organic photoreceptor has a drawback of being mechanically abraded with an image developer and a cleaner when used repeatedly in the electrophotographic process. In addition, as toner particles have smaller particle diameters due to requirements for high-quality images, cleaning blades have greater rubber hardness and higher contact pressure to better clean the photoreceptor. Unfortunately, this also accelerates abrading photoreceptors. Such abrasions of photoreceptors deteriorate electrical properties such as sensitivities and chargeabilities, and cause abnormal images such as image density deterioration and background fouling. When a photoreceptor is locally abraded, images having black stripes due to defective cleaning are produced. At present, photoreceptors are exchanged because of these abrasions and damages.
Therefore, it is essential to decrease the abrasion amount of the organic photoreceptor to achieve greater durability. Further, it is desirable for the organic photoreceptor to have a low surface energy to prevent a toner from adhering thereto, and to have good cleanability and transferability.
As methods of improving the abrasion resistance of a photoreceptor, (1) Japanese Laid-Open Patent Publication No. 56-48637 discloses a photoreceptor using a hardening binder in its surface layer; (2) Japanese Laid-Open Patent Publication No. 64-1728 discloses a photoreceptor using charge transport polymer material; and (3) Japanese Laid-Open Patent Publication No. 4-281461 discloses a photoreceptor having a surface layer wherein an inorganic filler is dispersed.
The photoreceptor using a hardening binder of (1) increases residual potential and decreases image density because of poor solubility of the binder with a charge transport material and impurities such as a polymerization initiator and an unreacted residual group. The photoreceptor using charge transport polymer material of (2) and the photoreceptor having a surface layer wherein an inorganic filler is dispersed of (3) have abrasion resistance to some extent, but which is not fully satisfactory. Further, the photoreceptor having a surface layer wherein an inorganic filler is dispersed of (3) tends to increase a residual potential and decrease image density because of a trap on the surface of the inorganic filler. The photoreceptors of (1) to (3) do not have satisfactory electrical and mechanical durability.
To improve the abrasion and scratch resistance of the photoreceptor of (1), Japanese Patent No. 3262488 discloses a photoreceptor with a protection layer made of a hardened multifunctional acrylate monomer. When a low-molecular-weight charge transport material is simply included in a surface layer, the low-molecular-weight charge transport material is not soluble with the hardened multifunctional acrylate monomer and the low-molecular-weight charge transport material separates and becomes a cloud in the surface layer. This causes deterioration of mechanical strength of the photoreceptor. Further, since the hardened multifunctional acrylate monomer is reacted in a surface layer including a polymer binder, the monomer is not fully hardened. Thus, it is not soluble with the binder and fails to cause a surface concavity and convexity of the resultant photoreceptor due to the phase separation when hardened, resulting in defective cleanability.
Japanese Patent No. 3194392 discloses a method of forming a charge transport layer using a coating liquid formed from a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond and a binder resin. The binder resin includes a binder resin having a carbon-carbon double bond and a reactivity with the charge transport material, as well as a binder resin having neither a carbon-carbon double bond nor a reactivity with the charge transport material. When a binder resin does not react with a charge transport material, the binder resin is not soluble with a hardened material produced by a reaction between the monomer and charge transport material. This causes a surface concavity and convexity of the resultant photoreceptor due to the phase separation when crosslinked, resulting in defective cleanability. Further, the binder resin prevents the monomer from hardening, and the monomer used in the photoreceptor is a difunctional monomer which has few functional groups and does not have a sufficient crosslinked density. Therefore, the abrasion resistance of the resultant photoreceptor is not satisfactory. Even when a binder resin reacts with a charge transport material, since the monomer and binder resin have few functional groups, it is difficult to have both a bonding amount of the charge transport material and a crosslinked density. The resultant photoreceptor, therefore, does not have sufficient electrical properties and abrasion resistance.
Japanese Laid-Open Patent Publication No. 2000-66425 discloses a photosensitive layer including a hardened positive hole transport compound having two or more chain polymerizing functional groups in the same molecule.
However, since the photosensitive layer includes a bulky positive hole transport material having two or more chain polymerizing functional groups, a distortion appears in the hardened compound and internal stress increases to cause roughness and cracking in the surface layer, resulting in insufficient durability of the resultant photoreceptor.
On the other hand, Japanese Laid-Open Patent Publications Nos. 57-35863, 62-75641, 63-61256, 63-73267, 64-35448, 2-189550 and 11-344818 disclose methods of including a variety of lubricative additives in an outermost layer to decrease a surface energy of an organic photoreceptor for the purpose of imparting good cleanability and transferability. However, since these photoreceptors include a lubricant in the photosensitive layer having an insufficient abrasion resistance, adherence of various materials can be prevented in initial stages, but cannot be maintained for long periods.
Japanese Laid-Open Patent Publication No. 2000-310872 discloses a method of including a hardening acrylic compound and a reactive acrylic siloxane compound in a protective layer, and Japanese Laid-Open Patent Publication No. 2001-166510 discloses a method of including a saturated hydrocarbon compound as a lubricant in a hardened surface layer. The former uses a hardening compound without a charge transportable structure and uses an electroconductive particulate metal oxide to control resistivity of the protection layer. Therefore, deterioration of resistivity is inevitable due to a water-absorbing property of the electroconductive particulate metal oxide, and the photoreceptor produces blurred images. In the latter method, a lubricant, i.e., the hydrocarbon compound is chemically bonded with a matrix material in a hardened photosensitive layer and the lubricant is taken therein to prevent bleeding out and to maintain a low surface energy. However, since the photosensitive layer includes a bulky positive hole transport material having two or more chain polymerizing functional groups, a distortion appears in the hardened compound and the internal stress increases to cause roughness and cracking in the surface layer, resulting in poor durability. In addition, the distortion in the photosensitive layer enlarges concavity and convexity on the surface of the resultant photoreceptor, resulting in a smaller contact area between the photoreceptor and contact members. Therefore, the original low surface energy is not exerted.
Accordingly, even a conventional photoreceptor having a crosslinked photosensitive layer chemically bonded with a charge transportable structure fails to have sufficient overall characteristics. Although a variety of inventions to lower the surface energy have been made, these changes do not satisfactory improve the durability, electrical properties and other properties.
Because of these reasons, a need exists for an electrophotographic photoreceptor with good cleanability, high durability and stable electrical properties for long periods of time.